The invention is generally related to radio transceiver circuits and particularly but not exclusively to pin diode switches used for switching radio transceivers. Known radio transceiver circuits with a receiver and a transmitter usually use one common antenna for transmitting and receiving radio signals. A switch connects either the input line of the receiver or the output line of the transmitter to the antenna.
This switch can be a single pole double throw type and is required to isolate the transmitter from the receiver in receive mode and to isolate the receiver from the transmitter in transmit mode. In its off mode it also provides a high impedance for matching. Due to its functions it is referred to as a transmit/receive switch or T/R switch.
There are a number of solutions for implementing the transmit/receive switch. The important considerations are size, cost, insertion loss and power consumption of the switches.
For example in mobile phone handsets it is common to use GaAs FET switches which are readily commercially available but are expensive and relatively large. They generally require two control lines to provide the bias voltages for turning on the transistors.
Another possible type of transmitt/receive switch is a PIN diode switch which is physically smaller and cheaper than transistor switches. A PIN diode switch presents a high impedance to RF frequencies unless a DC bias current is present. When a DC bias current is passed through the diode, the diode is turned on and provides a low resistance RF path between anode and cathode of the diode.
For the purpose of connecting an antenna to the receiver or transmitter the PIN diodes are typically coupled together with their cathodes. They are selectively biased by a bias current provided by a switchable power supply. If the power supply for one PIN diode is turned on a bias current flows through the pin diode and if this is e.g. the PIN diode in the receiver path a signal received by the antenna passes the PIN diode, which is now in its conductive state and enters the receiver input stage. The PIN diode in the transmitter path, which is not biased during receive mode, isolates the antenna from the transmitter output stage at the same time.
For this solution blocking capacitors are necessary to isolate the transmitter, the receiver and the antenna from the DC bias current. At least three RF chokes are needed for blocking the RF frequency signal from the common ground of the transceiver and from the power supply.
One further disadvantage of using PIN diodes the way described above is that an additional current of approximately 10 mA is required for biasing the PIN diodes. This additional current consumption can significantly reduce the standby and talk times of handheld radio communication devices like mobile phones which are powered by a single battery. Additionally two control lines and switches are required for the control of the bias currents.
One problem with PIN diode switches is that the isolation of the PIN diode switch may not be high enough when the PIN diode is not biased to repress the transmitted signal from the receiver input. Therefore it might be necessary to provide a negative bias current for the PIN diode in the receive path, but providing a negative voltage is also undesirable.
In accordance with a first aspect of the invention there is provided radio transceiver circuit comprising an antenna, a transmitter, a receiver, an antenna switching circuit for selectively connecting the transmitter and the receiver to the antenna, and a power source for both the transmitter and the receiver, the antenna switching circuit having two semiconductor elements, each having a conductive electrical state and a substantially non-conductive electrical state, one of the semiconductor elements being coupled between the antenna and the transmitter, the other of semiconductor elements being coupled between the antenna and the receiver, whereby the power source is operable to provide the current for powering the transmitter during operation of the transmitter and the receiver during operation of the receiver, current being sought by the transmitter setting the one semiconductor element to its conductive electrical state, current being sought by the receiver setting the other semiconductor element to its conductive electrical state thereby selectively coupling the receiver and the transmitter to the antenna.
The semiconductive elements are preferably PIN diodes.
The power supply (Vcc) for biasing the PIN diodes and supplying the transmitter and receiver with the needed biasing current is advantageously connected to the circuit node where the antenna is connected to the anodes or cathodes (dependent on the sign of the power supply) of the PIN diodes through a choke.
The choke which is coupled between the power source and the antenna is having a high impedance for RF signals and a low impedance for DC signals in order to provide the DC current for the PIN diodes, the receiver and transmitter circuit and block the RF signal from the power supply.
Furthermore the choke can be one of the matching components for the antenna which is used for providing the correct impedance matching for the antenna.
In a preferred embodiment of the invention the transmitter and the receiver circuit include open collector output stages. The power amplifier of the transmitter contains an open collector output stage and the bias current is feeding directly into the output of the power amplifier for biasing. This bias current is also used for biasing the PIN diode in the path between the antenna and the transmitter. This power amplifier could be advantageously the last power amplifier stage of the transmitter.
Also components of the receiver, for example a mixer of the receiver, could contain an open collector output stage and the mixer of the receiver requires the current which is used for biasing the PIN diode which is coupled between the antenna and the receiver.
In a further advantageous embodiment of the invention the transmitter is operated with a balanced output signal. This means that the power amplifier of the transmitter produces a first output signal with the half of the amplitude required for transmission and a second output signal also with the half of the amplitude required for transmission which is inverted to the first output signal. The second output signal is phase shifted by a phase shifting element, which can be for example a balun, and added to the first output signal to provide the required transmitter output signal.
The summation signal of the inverted and non-inverted transmitter output signal is then connected to the PIN diode in the transmitter path and the output signal of the transmitter is connected to the antenna during operation of the transmitter.
In accordance with a further aspect of the invention, there is provided a method of operating a semiconductor element switch of a transceiver, for selectively coupling an antenna to the transmitter and receiver, the semiconductor element switch having two semiconductor elements each having a conductive electrical state and a substantially non-conductive electrical state, the current sought from a power source by a transmitter of the transceiver during operation of the transmitter or by a receiver of the transceiver during operation of the receiver being used to set the respective semiconductor element into a conductive electrical state thereby connecting the antenna to the transmitter during operation of the transmitter and to the receiver during operation of the receiver.
In accordance with a yet further aspect of the invention there is provided a radio transceiver circuit comprising a transmitter having means for providing a balanced output signal with an inverted output signal and a non inverted output signal, and a receiver, the inverted transmitter output signal is phase shifted by a first phase shifting element and a second phase shifting element and combined with the non inverted output signal to provide an output signal, the first and second phase shifting elements are arranged such that the combined signal of the inverted and non inverted transmitter output signals is substantially zero at a node where the two phase shifting elements are coupled together, and said receiver is coupled to said node.
An embodiment of invention is obtained by using two phase shifting elements, i.e. baluns, instead of one phase shifting element in the transmitter path. The phase shifting elements are arranged in a way that the sum of the signals of the inverted and non-inverted transmitter output signal becomes substantially zero at the connection point of the phase shifting elements.
The input line of the receiver is connected to the connection point of the two phase shifting element.
While the transmitter is operating the signal at the connection point of the phase shifting elements is substantially zero and the receiver does not receive any signal this time even if it is connected to the transmitter path. While the transmitter is not operating a signal expected by the receiver can be obtained from the connection point of the two baluns.
In a preferred embodiment of this variant of the invention two PIN diodes for de-coupling the transmitter circuitry from the receiver circuitry are located within the inverted and non inverted output signal path of the transmitter providing a balanced output signal. While the transmitter is not operating these PIN diodes are in their non-conductive electrical state because the transmitter does not require a current for operation which bias the PIN diodes.
This invention facilitates a receive path from the antenna to the receiver without using any kind of switching or de-coupling element for de-coupling the receiver path from the antenna path.